Channel utilisation is not in the MIB, unfortunately.
I haven't checked very recently but last time I was reading the release notes for MiCC there was *still* no support for traffic stats on SIP trunks.
If you watch the SMDR stream you will be able to see when calls are on ISDN because the trunk numbers will look different.
Also you can use the "Expensive Route" option on ARS and callers will get a message on their phones when it has moved on to the next route.
If a SIP trunk has failed it will raise an alarm, that will be visible with SNMP or an email alert.
The only place I've ever managed to get channel utilisation statistics from is an upstream SIP gateway.